WD Passport for Mac External Hard Drive will not mount - can't fix or erase : PROBLEM SOLVED
Big Sur OS 11.2.3
Had this issue WD Passport for Mac External Hard Drive will not mount - But the Disk Utility could see the drive but could not fix or erase it (reformat).
Resolved the problem by downloading the WD disk utility for Mac:
Software and Firmware Downloads | WD Support (wdc.com)
Had to run the 'bad sector' feature a few times and it eventually passed (must have also fixed the bad sectors). End result was that the drive mounted and works fine now.
iMac 27″, macOS 10.13